This relates to apparatus for tempering glass sheets suspended in a vertical orientation and, in particular, to such apparatus that can readily accommodate changes in the size, shape and pattern of the sheets to be tempered.
The basic process of tempering glass is old and well known. Such process comprises heating a glass sheet above its annealing temperature and then rapidly cooling the surfaces of the glass sheet to set the latter while the center is still hot. This action provides glass sheets with a stress pattern in which the surface of the glass is stressed in compression and the center portion is stressed in tension.
A surface stressed in compression makes the sheet much stronger than untempered glass because glass is better able to withstand external forces when it is stressed in compression than when it is stressed in tension. Moreover, when the outer surface or compression skin of tempered glass is penetrated, the tension stresses within the glass cause it to shatter into a large number of relatively harmless, smoothly surfaced pieces. In contrast, annealed glass is fractured more easily, and when fractured, breaks into relatively dangerous, large, uneven jagged fragments.
In conventional glass tempering operations, the glass sheet is heated close to its melting point, and then quickly quenched by uniformly exposing the opposite surfaces of the heated glass sheets to streams of a fluid or gas, such as air, arranged to cool both surfaces uniformly and simultaneously. Typically, the fluid or gas is dispensed through two sets of nozzles, one set facing one surface of the curved glass and the other facing the opposite surface of the glass.
In the treatment of glass, it is desirable to assure uniform tempering of glass to avoid an uneven stress pattern in the compression skin. If unevenly tempered glass is fractured, this uneven stress pattern causes jagged fragments of glass which are more dangerous than the smoothly surfaced, uniform fragments of a broken sheet of uniformly tempered glass. In addition, unevenly tempered glass causes patterns of iridescence to form on the surface of the glass which are very annoying when viewed in reflection.
To temper glass uniformly, the cooling air must be evenly distributed on the glass surfaces. In conventional apparatus for tempering glass, this is accomplished by blasting air into an air distribution box and out through a plurality of uniformly spaced, elongated nozzles which are located a uniform distance from the adjacent glass sheet. To provide a uniform distance between the box and the sheet, the air distribution box has a fixed curved surface facing the glass sheet which has a shape conforming to that of a particular high production pattern of glass sheet. Unfortunately, such apparatus does not have the structural flexibility to provide uniform quenching of dimensionally varying glass of a particular pattern which may result, for example, from variations in the glass making process.
In current machines for tempering glass sheets mounted in a vertical orientation, the air distribution boxes ordinarily are mounted so that they only can move in a horizontal direction toward or away from the sheet of glass and cannot be oriented with respect to the precise vertical position of the glass sheet. Frequently, the curvature of the glass sheets, particularly those having multiple curvatures, is such that the sheet is not always suspended in an exactly vertical position; and there are variations from sheet to sheet in the angle of inclination of the sheets from the vertical. As a result, horizontal movement of the air distribution box does not make it possible to assure a uniform distance between the nozzles on the opposed air distribution boxes and the surfaces of the glass sheet between them. Consequently, there is an irregularity in the tempering of the glass, with the areas closest to the nozzles exeriencing more tempering. As a result, to assure sufficient tempering of the entire surface of a glass sheet, it is necessary to use more tempering gas or fluid to assure that all portions of the glass sheet are sufficiently tempered, with a resulting increase in operating cost and variations in the amount of tempering.
These problems are particularly acute when tempering glass as thin as three millimeters or less. For thicker glass, it is possible to position the nozzles at relatively large distances from the glass, so that deviations in the separation between the nozzles and the glass surface are not very bothersome. However, for glass as thin as three millimeters or less, the distances between the nozzles and the glass are considerably smaller, the blowing pressures are greater, and any variations in the separation between the nozzles and the glass produce much greater irregularities in tempering and in some instances make it impossible.